The present invention relates to electrochemical half cells, and in particular, it relates to an electrolyte within the half cell.
In the prior art, electrochemical cell electrolytes are typically a liquid, gel, or a slurry. Each type of electrolyte has advantages over the other types for a particular use. However, all the prior art electrolytes share some problems.
Dehydration of the electrolyte is a problem. Prior art electrolytes need a high water content. However, water loss to the atmosphere or to the solution being sampled occurs through the cell junction. Cell stability and the life of the cell are reduced by the water loss.
The stability of an electrochemical cell also depends on a fixed ionic concentration at the electrode. Prior art aqueous solution electrolytes easily lose ions through the cell junction due to the high physical mobility of the electrolyte within the cell and the high mobility of the ions within the electrolyte.
Another problem with the prior art half cells are that the electrode within the cell may become poisoned by the sample solution. It is desirable for cell stability that the electrode remain a specific metal/metallic salt. However, sample solutions contain ions that will alter this relationship within the electrode. Such ions should be excluded from the cell, just as the electrolyte ions should be retained within the cell. This problem has been addressed to some extent in the prior art by the use of multiple junctions, each with its own electrolyte to slow down the diffusion of the poisoning ions from the junction at the sample solution to the metal electrode within the cell.
Electrochemical cells also have a problem with a voltage potential developing across the junction. The voltage potential occurs from at least two sources.
The first source is a build-up of salt at the junction. Most aqueous electrolytes contain excess salt to make up for ion loss to the sample solution. Occasionally, the excess salt crystallizes over the junction and can create a very significant error, as great as 50 to 100 mV.
The second source is pressure change that results in the sample solution and/or the electrolyte moving in or out of the cell through the junction. Such movement results in ion species and concentration changes across the junction. Such movement can generate significant error causing potentials.
In an article entitled "A Solid Polymer Electrolyte Internal Reference Electrode for High Temperature Aqueous Systems" by Hettiarachchi a reference electrode having a solid electrolyte is discussed. The electrode is prepared by mixing commercially available high temperature epoxy with an appropriate amount of Al.sub.2 O.sub.3 filler and a 0.1M KCl solution to a reasonably viscous mix to immobilize the chloride ions. The mixture is placed within a tube having an Ag/AgCl element and the open end of the tube is plugged with a porous zirconia junction to minimize contact between the polymer and the solution to be sampled. Although this electrode is an advancement in the art, the electrolyte is porous in nature due to the ceramic Al.sub.2 O.sub.3 particles that are bound within the epoxy. In addition, the electrolyte is internally wetted with the KCl solution and is therefore subject to dehydration and ion depletion.